The present invention relates to a bipolar plate for a fuel cell and a cooling plate for a fuel cell. The fuel cell bipolar plate according to the present invention has a cathode side and an anode side, wherein the cathode side comprises one or more flow channels in the form of serpentine grooves comprising by-pass channels. In analogy herewith, the cooling plate according to the present invention comprises a cooling side, wherein the cooling side comprises one or more flow channels in the form of serpentine grooves comprising by-pass channels. The bipolar plate and the cooling plate according to the present invention are particularly useful in the type of fuel cell stacks, wherein oxidant gas is used as reactant gas for operation of the fuel cells. Such fuel cell comprise inter alia PEM fuel cells and SOFC fuel cells.
Fuel cells are devices that provides for clean and relative efficient conversion of matter into electrical energy and heat. A range of different technologies have been developed within the last couple of decades, each employing its own principle, type of reactants, optimum operation conditions etc. One technology which has gained particular interest in recent years is the so-called PEM (proton exchange membrane) fuel cell. Another technology which has gained particular interest in recent years is the so-called SOFC (solid oxide fuel cell) fuel cell. The background of the present invention will know be explained in respect of a PEM fuel cell.
A PEM (proton exchange membrane) fuel cell comprises an anode and a cathode and a proton exchange membrane interposed there between. The proton exchange membrane comprises a catalyst on the side facing the anode as well as on the side facing the cathode. The principle of a PEM fuel cell is that supplying hydrogen to the side of the membrane facing the anode by virtue of the catalyst on the side of the membrane facing the anode results in the chemical reaction:Anode reaction: H2→2H++2e−  (1)
The anode is made of an electrically conducting material and thus transports the electrons generated on the anode side of the membrane, whereas the protons generated on the anode side of the PEM-membrane diffused through the membrane.
On the cathode side of the membrane oxygen (or air) is supplied. If an electrical load is connected between the cathode and the anode of the cell so as to form an electrical circuit, the electrons generated at the anode flows through this load to the cathode. The oxygen supplied to the cathode side of the membrane by virtue of the catalyst on the side of the membrane facing the cathode reacts with the protons which have diffused through the membrane and the electrons flowing to the cathode according to the following chemical equation:Cathode reaction: O2+4H++4e−→2H2O+heat  (2)
Hence, the net reaction taking place in a PEM fuel cell is:2H2+O2→2H2O+electrical power+heat  (3)
One single cell is capable of generating a voltage of typically 0.5-1V. In order to achieve higher voltages for fuel cells, a number of single cells are usually connected in series in a so-called fuel cell stack. A fuel cell stack is for the sake of economy often designed in a way that integrates the cathode of one fuel cell with the anode of an adjacent fuel cell of the corresponding stack. This is achieved by employing so-called bipolar plates. A bipolar plate is a plate which has two sides, one of which functions as an anode for one fuel cell, and the other of which functions as a cathode for the adjacent fuel cell in the corresponding fuel cell stack.
In order for a bipolar plate to be efficient, it must be assured that a constant supply of oxidant gas is delivered to the cathode side of the membrane and therefore also to the cathode side of the bipolar plate. Furthermore it must be assured that the oxidant gas supplied to the cathode side of the bipolar plate is distributed well over the surface of the cathode side of the bipolar plate. This in turn requires that oxidant gas is supplied at a relative high pressure from an outside source, such as the ambient air and into the inlet manifold connecting the cathode side of each bipolar plate of the fuel cell stack to the oxidant source.
On the basis of the above considerations much research and development of PEM fuel cells has in the recent years focused on the specific physical design of the fuel cell and in particularly on the physical design of the bipolar plates.
Due to the heat generated at the cathode of a fuel cell cooling means are needed to cool the cathode part of the fuel cell. In some designs of fuel cells the cathode side of the bipolar plate provides for supplying oxidant gas to the fuel cell as well as cooling the cathode side of the fuel cell by virtue of supplying more oxidant gas to the cathode side than is actually needed in relation to the stoichiometric amount of hydrogen being supplied and “consumed” at the anode side of the bipolar plate of the fuel cell. Other designs comprise distinct cooling plates which are not an integral part of the bipolar plate of the fuel cell.
French Patent application FR 2891090 discloses a fuel cell with bipolar plates having serpentine paths on their side. In order to press fluid through these channels, relatively high power consumption is used for pressing the gas through the channels, which is disadvantageously.
Japanese patent document JP 2003 100319 by Kino Yoshitaki assigned to Toyota Motor company discloses a fuel cell for low-temperature operation. The gas is moisture enriched in order to prevent too dry conditions for the membrane. The necessity for moisture implies a temperature below the boiling point. However, there is a certain risk that the channels of the cooling fluid are blocked with droplets from the moisture. For this region, by pass channels are provide with a resin that swells with increasing degree of moisture. When the degree of moisture increases, the by pass channels are blocked, and the flow speed increases in the remaining channel which reduces the risk for droplet formation and consequential blocking of the channel. The disadvantage of such a system is the relatively high power consumption used for pressing the gas through the channels at high speed. Such resin-filled by-pass channels are not used for high temperature fuel cells, where the gas is dry and where uptake of moisture is not necessary.
By pass channels for preventing condensation or at least minimising the risk for flow reduction by condensation in low-temperature fuel cells are also disclosed in Japanese patent applications JP 2001 126746 and JP 2006 351222, in US patent application No. 2004 011274.0, and in International patent application Wo2007/088832.
Accordingly numerous different designs of bipolar plates have been disclosed in the art. However, although a substantial amount of these designs fulfil the technical requirement of supplying sufficient oxidant gas to the cathode side of such plates, they all suffer from the disadvantage that in respect of bipolar plates, the specific design requires much energy for supplying the oxidant gas from the outside source via the inlet manifold into the cathode side of the bipolar plates of the fuel cell stack; and in respect of cooling plates, the specific design requires much energy for supplying the coolant fluid from the outside source via the inlet manifold into the cooling side of the cooling plates of the fuel cell stack